<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        var fan = prompt('请输入你的成绩');
        if (fan >=90) {
            alert('A');
        } else if (fan >=80){
            alert('B');
        } else if (fan >=70){
            alert('C');
        } else if (fan >=60){
            alert('D');
        }else{
            alert('E');
        }
    </script>
</head>
<body>
    
</body>
</html>